docs: def rosen(self, x, alpha=1e2):
          """Rosenbrock test objective function"""
          x = [x] if isscalar(x[0]) else x  # scalar into list
          f = [sum(alpha * (x[:-1]**2 - x[1:])**2 + (1. - x[:-1])**2) for x in x]
          return f if len(f) > 1 else f[0]  # 1-element-list into scalar